May 25, 2009 10:04 a.m.

I'm helping a friend set up a 4-link on a Model-A based street rod he's building. Are there any good online references, or real-world advice out there?

The only thing I remember is the front links on either side should be closer vertically than the rears to keep the pinion angle in line.

patgizz Dork
May 25, 2009 2:11 p.m.

what kind of 4 link?

on my belair i've got the lower links parallel to the ground and the upper links pointed down making the imaginary line where the links would meet, if extended, somewhere up around the firewall. it makes for a great off the line launch without pulling the nose up. i'm sure a lighter car would need set up differently. smoothest ride comes from all 4 bars being parallel to the ground so it all moves evenly together.

i was taught to install the lower links and adjust them, then install top left one and set pinion angle. recheck lowers and adjust if necessary then top left again, then just install the top right one where the holes line up.

Jensenman SuperDork
May 25, 2009 5:29 p.m.

The layout of the links will greatly affect anti squat, roll steer and brake hop.

Generally, the longer the link the better to help reduce brake hop. If the lower links point up at the front this adds anti squat, it could even make the rear of the car rise on acceleration.

If all 4 links are parallel as viewed from above or below, you will need a Panhard rod or Watts link to locate the axle. Even though that means adding one extra link (the Panhard) it's probably the simplest way to lay it out.

It's possible to run the lower links straight forward and angle the upper links so that a panhard etc is not needed but now you start running into roll steer characteristics and you have to have a whopper crossmember to feed the acceleration and braking loads into. If you decide to go this way, the links need to start near the differential 'pumpkin' and then angle outward.

As a general rule, the longer the links the less roll steer it will have. You want roll understeer, not roll oversteer BTW.

Herb Adam's Chassis Engineering has some real world practical advice on how to lay it out.

May 25, 2009 7:09 p.m.

Thanks. These will be parallel viewed from above & we will have a panhard...or maybe a watts link.

I guess what I'm specifically looking for is the math to calculate the spacing of the front links, relative to the rear links, needed to keep pinion and u-joint angles within ~5* or less.

Jensenman SuperDork
May 26, 2009 6:22 a.m.

Um, I'm confused. Front and rear links? I'm thinking all 4 links pointing forwards. Like this:

BTW, for a street driven car I'd avoid that angled 'cross link' and use a Panhard or Watts only.

aussiesmg Dork
May 26, 2009 7:09 a.m.

If I can butt in on this thread, is there any reason why the control arms cannot run backwards? I ask due to space constraints.

Jensenman SuperDork
May 26, 2009 7:45 a.m.

IMHO, it can be done. The things that would give me pause: the links are now in 'compression' rather than 'tension' and could be more prone to bending. Metal is always more resistant to bending in tension than compression. Also, it's hard to visualize but instead of the axle being 'dragged' over bumps it's now being 'pushed'. Probably the closest I can come is the Earles fork that was popular on dirt bikes for a few years, like this:

May 26, 2009 11:19 a.m.
Jensenman wrote: Um, I'm confused. Front and rear links? I'm thinking all 4 links pointing forwards. Like this: BTW, for a street driven car I'd avoid that angled 'cross link' and use a Panhard or Watts only.

The front mounts of the rear links. Sorry.

Just like your picture above - how do I know how far apart(vertically) to space the front link pivots? Do I only need to be worried about the anti-squat function, or do I need to worry about how that spacing affects pinion angle throughout travel?

44Dwarf Reader
May 27, 2009 10:03 a.m.

Loads of modifeds now use two forward and to backward 4 link set ups but its do to space needed theres no real advantage that i've heard of.

With the set up above you MUST use a latteral locater (panhard of Watts) due to the hiem joints in the bars. even if you angle the bars in (side ways) the rear will float. Stock Mustanges use the top bar pused in but there on bushings and they bind like heck.

IMO Pinion angle is for keeping the drive shaft happy and has nothing to do with forward bite bar angle do that.

Most find with the lower bars set 3 to 5 deg up at the body they need uppers 8-15 deg down at the body and adjust for bite and brake hop. On the dwarf i'm turning left only and use a 3 link unit my left lower is @ 5deg and ring lower is @ 3Deg the uper 3rd link is 10deg. this helps my roll steer and lifts the back slightly under power planting the tires.

44

Jensenman SuperDork
May 27, 2009 10:31 a.m.

Okay, now I understand. About my example below: keep in mind I am simply adapting a different axle to the car I am currently building not starting from scratch, so I had to run a shorter upper link due to the way the car's unibody is built.

The way I did my 3 link: I raised and leveled the car, then set my axle on stands below it with the pinion flange at 90 degrees to the body. The important part: the axle has to be at the correct ride height. I calculated my link lengths from that. I have enough adjustment in the links to change the pinion angle up or down as needed.

Pinion angle change through travel is most affected by whether the links are the same length. If the top link is a lot shorter but the links are parallel as viewed from the side, the pinion will angle downwards as the axle rises. If the lower links are a lot shorter, the reverse will happen. This is also a function of how much travel the axle will have, it's going to be a lot more pronounced on something whith a whole lot of suspension travel (rock crawler etc) than, say, a road race car with a total of 4" of travel.

Since you are starting from scratch, I'd recommend making the links as long as possible and keep them as parallel as possible when viewed from the side. 44's advice about angled links vs Panhard is good, it's a lot easier to do the PH and get things right. His angle recommendations sound good too.

May 27, 2009 1:57 p.m.

Thanks guys!

Yes, we're definitely using either a panhard or watts. I'll work out the side angles based on ~25% anti-squat and just worry about getting the pinion angle set static.

44Dwarf Reader
May 27, 2009 6:03 p.m.

My links are short too at around 17 inches for the bottoms and around 11 for the top link both uper and lower attach to the axle housing behind the axle center line so they could be as long as possible. Think Herb said 2 inches but mine ended up at 1.5 there abouts.

http://www.auto-ware.com/autoware-bin/tech.pl?#15235

click on rear supention demo.

I have a few of Johns programs there the best i've used yet.

Slyp_Dawg New Reader
May 27, 2009 6:36 p.m.

just wondering, but on the subject of panhard versus a watts linkage, which would be better? panhard bars seem to be much more popular, but I don't know if that's due to performance or whether it's down to the fact that it's much easier to build hardware for

May 27, 2009 6:53 p.m.

Purely based on how they function, a Watts Link is better due to the fact it will locate an axle latterally without allowing any side-to-side deflection. A Panhard will force the axle toward the anchored side as it travels further into droop, or bump.

However, like everything else regarding tuning, the reality is there are compromises. A Watts Link requires more pieces, which means more weight & complexity, plus there is a finite limit of travel with a Watts Link. A Panhard bar is much simpler, and correctly designed will offer minimal, unnoticeable, deflection. It will also still allow greater axle travel should something go horribly wrong.

44Dwarf Reader
May 27, 2009 7:21 p.m.

It also depends on the type of racing. Most aspalt car mount the panhard to chasis on the right side and dirt mounts chassis to the left side. Why you ask? Roll center change aspalt we like the roll center to drop in the corner and dirt they want it to go high the help get the back end to swing around. Watts linkage would not change the local so theres less dynamic tuning availible.

44

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